Energy curable laminating adhesives meet varied needs

Paul Boughton

Sean Des Roches outlines four different laminating adhesives with different end properties to suit industry needs.

Over the past decade, laminating adhesives have become a relevant sector of energy curable technology. Initially disregarded as too expensive and not technically advanced enough for laminating applications, energy curable formulations become a major player in the converting and packaging industry especially for speciality applications. Recently, development work into the realm of food packaging has begun to build profitable businesses for those willing to use this technology.

Laminating adhesives generally bond clear laminate plastic films to various plastic films, paper or metallic foil substrates. For this purpose, curing by UV and adry' laminating adhesives are usually adequate. Upon curing, this type of adhesive offers a non-tacky film that bonds sufficiently to the respective substrates and achieves excellent T-Peel properties. However, when a specific application requires laminating opaque substrates or films that exhibit adherence difficulties, EB curing or PSA-type (wet) laminating adhesives may be necessary. After cure, this type of adhesive results in a tacky PSA-like adhesive. EB cure is not negatively affected by opaque substrates, and PSA-type laminating adhesives can be cured prior to the lamination.

Rahn's goal and interest lies in supporting industry needs with functional formulations that serve as starting points for specific projects. We have formulated four different laminating adhesives with different end properties to suit industry needs.

Each adhesive offers the preceding properties at a film thickness of 0.3mil (7.7µm) on substrate that has been corona treated to 42dynes/cm and cured through the laminate film using a 300wpi (120W/cm)Hg bulb. It is not possible to have one formula for each specific substrate and specially the combination of those. One of the most versatile formulas is the wet type LAUS03003. T-Peel Properties (PSA-type Laminating Adhesive (US03003): T-peel values are dictated by the elastic and adhesive properties of a laminating adhesive.

Monofunctional Urethane Acrylate exhibited the highest peel results with each plastic film combination tested, resulting in exceptional elongation properties (384percent) and good cohesive strength. This balance of flexibility and cohesion coupled with excellent adhesion attributes produces high peel values in a laminating adhesive. An increase in monofunctional Urethane Acrylate will raise the adhesive's T-peel performance.
In general: The best two monomers for T-Peel strength properties were the monofunctional Aliphatic Urethane Acrylate and the IBOA. Best to use in combination.

Lower T-Peel but good cutting power has EOEOEA.

Low cost (A), with PET (B) and with non-PET (C) type LA. In all three formulations, an increase in the overall oligomer/monomer ratio, despite increasing the viscosity, will offer enhanced T-Peel performance.

For example: Additional organic filler cut in TPGDA (Genomer* 6058/TP) or small amounts of Inert Resin cut in TPGDA (Genomer* 6083/TP) will benefit the T-Peel properties of Formulation A. With regard to Formulation B, if a very rigid PET is the substrate of choice, additions of Difunctional Aliphatic Urethane Acrylate combined with IBOA, will assist in flexibilising the adhesive and offer improved adhesion properties. For each formula, an increase in the amount of Adhesion Promoter will increase the T-Peel performance through improved substrate adhesion. Finally for the European market, Acryloyl Morpholine (ACMO) has been observed as an efficient diluent for laminating adhesives with excellent adhesive properties.

And for viscosity: For Formulation A, EOEOEA is the most efficient reactive diluent for reducing viscosity. For all three formulations, DPGDA, HDDA and ACMO (for Europe) offer exceptional viscosity cutting properties while maintaining good adhesion performance. Trifunctional TMP(EO)3TA and trifunctional Polyether Acrylate offer higher viscosity alternatives with increased reactivity and good adhesion properties, especially regarding trifunctional Polyether Acrylate on treated PE and OPP.

Increasing the crosslink density the peel strength, elasticity and adhesion will significantly be reduced. Be aware, that the cure window of a PSA formula has to be matched carefully.

Following changes could be done without dramatic influence of the adhesives properties, eg in FormulationC, replacing a portion of the monofunctional Acid Functional Acrylate Oligomer with trifunctional Aliphatic Urethane Acrylate instead of hexafunctional DPHA will help to maintain adhesive flexibility.

Replacing used Photoinitiator with a more efficient photoinitiator will increase adhesive reactivity without too negative influence of the adhesive's properties.

Especially PET and other plastic films may absorb UVB and C. We recommend to add more efficient photoinitiators in this case in combination with a Fe-doped bulb.

To summarise, the four different LA-formulas are good for: US03003: Low viscosity PSA-type (wet) laminating adhesive with adhesion to a wide variety of substrates including treated PET, PE, OPP, metallised foils and paper. US03012A: Low cost low viscosity laminating adhesive for treated OPP, PE and paper. US03012B: Low viscosity laminating adhesive for plastic film, paper and AL foil in combination with treated PET. US03012C: Versatile low viscosity laminating adhesive for treated non-PET Film, metallised OPP, AL foil and paper.

Sean Des Roches is Laboratory Manager Lab USA, Rahn Group, Zürich, Switzerland.